A study of the localization of the calcium binding protein calretinin (CR) in cells of the reticular formation (RF) of the rat and the colocalization of these CR cells with subtypes of glutamate receptors has been completed using a goat antibody made to CR. CR was found in subpopulations of cells in all subnuclei of the RF with a prominent colocalization with GluR2/R3 and GluR5/R7 glutamate receptors. This was particularly apparent in the gigantocellular cells which are believed to mediate the acoustic startle reflex. A study was initiated to examine the interaction of CNS drugs and sensory stimulation on calcium binding proteins and other cellular markers in auditory nuclei. Control rats exposed to intermittent tones showed an initial increase in locomotor activity which habituated over repeated tone presentations while morphine (10 mg/kg i.p.) treated rats showed increased activity during tones across all trials with no indication of behavioral habituation. Examination of a marker of immediate early gene activation (phospho-CREB) revealed effects of the tone and morphine combination in the reticular formation and in auditory nuclei. A comparative analysis of the localization of calcium binding proteins revealed high amounts of CR expression in a well defined population of cells (spherical bushy cells) of the anteroventral cochlear nucleus of guinea pig. This result is consistent with immunohistochemical studies of human, primate, cat and dog, but not in rats or mice. The latter rodent species instead express parvalbumin in these cochlear nucleus cells. These results suggest that CR and parvalbumin may have interchangeable calcium buffer functions in the spherical bushy cells.